Method for fabricating self-moderating nuclear reactor fuel element



J. A. DUDEK ETAL 3,197,389 METHOD FOR FABRICATING SELF-MODERATINGNUCLEAR REACTOR FUEL ELEMENT Original Filed Dec. 15, 1959 July 27, 1965INVENTORS JOSEPH A. DUDEK EDWARD L. REED 3 4 IiTOIfi United StatesPatent 3,197,389 METHOD FUR FABRECATING SELF-'MGDERATKNG NUCLEAR REACTQRFUEL ELEMENT Joseph A. Dudeh and Edward L. Reed, Woodland Hiils, Calih,assignors, by mesne assignments, to the United States of America asrepresented by the United tates tomic Energy Commission Originalapplication Dec. 15, 1959, Ser. No. $59,815.

Divided and this application May It 1963, Ser- No. 285,175

1 Claim. (Ci. ass-rs This is a division of application Serial No.859,815, filed December 15, 1959, now US. Patent 3,170,847 issuedFebruary 23, 1965. Our invention relates to a method for the manufactureof a self-moderating nuclear reactor fuel element.

The theory, construction, and operation of nuclear re actors is wellknown in the art and may be found in such references as Principles ofNuclear Reactor Engineering by Samuel Glasstone, published by D. VanNostrand Company, Inc., Princeton, New Jersey, First Edition, 1955; TheProceedings of the Geneva Conference on the Peaceful Uses of AtomicEnergy held in Geneva, Switzerland, August, 1955, available for sale atthe United Nations Bookstore, New York City, New York; and US. Patents2,708,656 and 2,714,577, Fermi et al. When a nuclide undergoes fissionin a nuclear reactor, a larger quantity of the energy liberated appearsin the form of heat. In the case of a power reactor, the heat liberatedis used to perform useful work. A major problem in any reactor powersource is the fuel and moderator element. One problem encountered is todesign a fuel element such that the temperature difference between thecoolant which flows around the element and the element center is reducedto a minimum at any designed heat flux. Another problem is to design afuel element which will result in reduced radiation damage to themoderator material employed.

It is, therefore, an object of this invention to provide a compositionof matter which can be modified to serve as a nuclear reactor fuelelement. It is also an object of this invention to provide a reactorfuel element assembly which performs the triple function of moderator,end reflector, and fuel. Another object is to provide a fuel elementwhich will have the characteristic of exhibiting a reduced temperaturedifference between the coolant flowing around the element and theelement center at any particularly desired heat flux. Another object ofthis invention is to provide a fuel element which is less susceptible toradiation damage of the moderator material. It is also an object of thisinvention to provide a process for the preparation of a compositon ofmatter which can be modified to serve as a nuclear reactor fuel element.Still other objects of this invention will become apparent from thediscussion which follows.

The objects of this invention are accomplished by an article ofmanufacture comprising an electrically conductive core materialencompassed by a layer of electrically conductive material containinginterspersed therethrough a radioactive element-containing substance, Anexample of such an article is a cylindrical core of metal hydride with alayer of a nuclear reactor fuel such as uranium oxide in a matrix of agood heat-conducting material such as copper on the surface of the core.Preferably, the uranium oxide impregnated copper is in intimate contactwith the metal hydride central cylinder, which contact can be achievedby bonding by methods well known in the art. For example, when the outerlayer matrix is copper and the central moderator material is zirconiumhydride, sufiicient contact is obtained between the copper and thezirconium hydride by merely plating a thin layer of copper on the corein a copper strength solution.

Eddifib Patented July 27, 1965 Although the above description has beenmade with reference to a cylindrical core, the shape of the article isnot intended to be limited to that of a cylinder. The article of thisinvention can be in the form of flat plates, or any other outer shape.

The amount of electrically conductive metal employed to bind togetherthe radioactive fuel particles need only be sufficient to impart a goodheat transfer characteristic to the fuel and matrix layer. This isaccomplished by a composition in which the atom ratio of radioactivefuelto-binding metal in said layer is from about 1:1 to about 1: 12. Formore satisfactory transfer, it is preferred that the atom ratio ofradioactive fuel-to-binding metal be in the range of from about 1:3 toabout 1:7. For example, good heat transfer characteristics are obtainedin an uranium oxide-copper layer when the atom ratio ofuranium-to-copper is substantially 1:53.

In order to protect the article from corrosion, it may have an outsidelayer of a protective metal such as nickel. For example, a nuclearreactor fuel element may be composed of a zirconium hydride core onwhich is deposited a layer of uranium oxide in a copper matrix, andthis, in turn, is covered with a protective layer of nickel.

Various moderator core materials may be used in the manufacture of anuclear reactor fuel element. Preferably, the material is composed ofcarbon, a metal, hydrides or oxides of a metal, or mixtures of two ormore of these components, wherein the carbon or metal has a neutroncapture cross section in the range of from about 0.0032 to about 2.5barns. The lower limit represents a neutron capture cross section ofcarbon whereas the 2.5 barns is the neutron capture cross section formolybdenum and ruthenium. Non-limiting examples of elements that can beused as a moderator include carbon, beryllium, magnesium, aluminum,iron, zinc, germanium, rubidium, strontium, yttrium, zirconium,titanium, niobium, tin, barium, ceritun, lead, and bismuth. Hydrides ofthese elements such as zirconium hydride, yttrium hydride, and titaniumhydride as Well as oxides of these elements such as beryllium oxide,magnesium oxide, and alumina can also be used. Examples of carbidemoderator are zirconium carbide, titanium carbide, and yttrium carbide.

The radioactive elements that can be employed are the radioactiveelements which are well known to those skilled in the art. When thearticle is a fuel element for a nuclear reactor the radioactive elementsthat can be employed are those of Series Vii of the Periodic Table ofElements which includes uranium, thorium, plutonium, etc. The oxides ofthese elements such as uranium oxide, thorium oxide, and plutonium oxidecan also be used. In addition, the hydrides of the elements, as well asthe hydride of alloys containing such radioactive elements, can beemployed. Non-limiting examples of the hydrides are the hydrides of analloy of uranium, zirconium and yttrium; and the hydrides of an alloy ofuranium, molybdenum and thorium. Still other compositions which can beused as particle components to be dispersed in a suitable matrix arecarbides such as uranium carbide, thorium carbide, and plutoniumcarbide. A mixture of two or more of the above components may also beemployed.

The metals or combination of different metals which can serve as amatrix in which the radioactive nuclides or elements are dispersedinclude such elements as copper of Group 13 of the Periodic Table of theElements; beryllium, magnesium, zinc, and strontium of Group H; aluminum, yttrium, and lanthanum of Group III; titanium, zirconium,germanium, and lead of Group lV; vanadium, niobium, tantalum, andbismuth of Group V; chromium, molybdenum, and tungsten of Group VI; andiron, nickel, palladium, and platinum of Group VIII of the PeriodicTable of the Elements as shown in the Handbook of Chemistry and Physics,pages 392 and 393, published by snezess the Chemical Rubber PublishingCo., Cleveland, Ohio, 37th Edition, 1955. Alloys of these compositionsmay also be used as a matrix to bind the radioactive nuclidecontainingparticles. The requirement is that the matrix have a good heat transfercharacteristic so that the heat produced by the radioactive elements dueto radiation or fission will be readily conducted to the outer surfaceof the article where it can be taken away by suitable heat transfermedia.

The thickness of the electrically conductive matrix and radioactivematerial varies from about 0.0002 inch to about 0.1 inch and thicker asdesired, depending on the service in which it is to be placed.

The mat rial used to coat the article in order to protect it frompossible corrosion can be any of the metals named above that serve as amatrix for the radioactive nuclide, of which such elements as nickel,chromium, steel, molybdenum, and tantalum are found to be suitable. Theprotective coating varies in thickness from about 0.0001 inch to about0.1 inch.

In general, the process by which the article of this invention isprepared comprises providing an electrically conductive core materialand electrophoretically depositing a radioactive substance of a particlesize of rom about 0.3 to about 100 microns in diameter on the corematerial in a layer of predetermined thickness or until theelectrophoretic deposition substantially ceases. A particle size of fromabout 0.3 to about 0 microns in diameter is preferred in order tominimize the danger of precipitation. This provides a layer of porousradioactive material on the electrically conductive core material. Anelectrically conducting matrix material of the type mentioned above isthen deposited electrolytically in the pores of the electrophoreticallydeposited radioactive substance so as to provide a matrix of conductivematerial containing interspersed therethrough the radioactive substancematerial. Any electrically conductive material such as copper, forexample, may first be plated out on the core material to form a layer offrom about 0.00005 inch to about 0.1 inch or more thick in order to makethe core material a better electrode for the electrophoretic depositionof the radioactive particles. The cycle of electrophoreticallydepositing the radioactive-containing particulate substance followed byelectrolytic deposition of a metal or alloy may be repeated any numberof times dependent upon the thickness of conductive material containinginterspersed radioactive material desired. Finally, a layer ofprotective metal is electrolytically deposited on the surface of thearticle, that is, deposited on the suriace of the layer made up of theelectrically conductive matrix containing the radioactive material. Thissurface layer is composed of any one of the metals or alloys of metalsused to serve as the matrix. The purpose of the surface layer is toprotect the article from corrosion and, therefore, a substance such asnickel is found to be suitable in this instance.

A non-limiting example of the article of manufacture of this inventionis shown in the accompanying drawing. PK}. 1 shows thecylindrically-shaped fuel element made by the process of this invention.FIG. 2 is a radial crosssectional view of the fuel element taken alongline 2 2 of FIG. 1, While PK}. 3 is a segment of an axial crosssectionalview taken along line 3-3 of PEG. 2. The segment shown in FIG. 4 is anenlarged version of the segment shown in FIG. 3 with a modified layerstructure.

In FIG. 1, the cyllndrically-shaped fuel element l1 consists of arod-shaped central portion 12 and end cap pieces 13 and 13 at both endsof the central body section 12. The end pieces are welded to the centralbody section at scams and lid. The element is made up of a central corematerial 15 encased in a layer 16 of electrically conductive metalcontaining interspersed therethrough a nuclear reactor fuel-containingmaterial. Covering the latter layer, is a sheath 1'7 of a protectiveelectrically conductive metal.

All

FIG. 2 represents a radial crosssectional view of the cylindrical fuelelement of FIG. 1 taken along line 2-2. In this figure, the fuel element11 is seen to be made up of a central rod-shaped moderator core material15. Number 1.5 and 1? refer to parts as described in reference to FIG.1.

An enlarged segment of a cross-sectional view of the fuel element ofPEG. 2 taken along line 33- is shown in FIG. 3. This figure shows thecore 15, the nuclear fuelcontaining layer 16, and the cladding 1'7.

4 is enlarged version of the segment shown in PEG. 3. The components aredistorted with respect to relative dimensions for purposes ofillustration. FIG. 4 illustrates a modified version of the element inthat there is a thin layer of electrically conductive material bethe coe 15 and the metal oxide-containing layer 16.

The article and process of this invention are more fully described inthe non-limiting examples which follow.

EXAMPLE I A slurry composed of five parts of powdered uranium dioxide inabout eight parts of isopropyl alcohol were added to a ball mill and themill operated for a period of 72 hours until the particles of uraniumdioxide were of a size of from about 0.3 microns to about 50 microns indiameter. After 72 hours of grinding in the ball mill, the contents wereremoved to a storage vessel and parts of additional isopropyl alcoholWere added. The mixture was stirred until uniform throughout. To themixture was then added 0.1 part of concentrated HCl to serve as anelectrolyte and the mixture agitated until a uniform composition wasobtained. The uranium oxideisopropyl alcohol mixture was then added toan electrolytic cell in which the container, made of steel, served asthe cathode and a cylindrical rod of zirconium hydride served as theanode. The zirconium hydride used as the anode was prepared bysubjecting zirconium massive metal of the required shape to highlypurified hydrogen gas in a furnace at l700 F. The zirconium hydride was4.592 inches long and had a diameter of 0.4001 inch, and Weighed 61.1926grams. The solution was kept in constant agitation so as to keep theuranium oxide uniformly distributed throughout the isopropyl alcohol. Apotential of substantially 1200 volts was maintained across the cellelectrodes consisting of the zirconium hydride and the container Walls.The potential caused the deposition of uranium oxide on the zirconiumhydride electrode. The deposition continued for approximately fiveseconds after which time the rate of deposit was negligible. Thepotential source was disconnected from the cell, the zirconium hydrideelectrode removed, rinsed with Water, dried with alcohol, and thealcohol permitted to evaporate. The electrode with the uranium oxidedeposit was then weighed and measured. It was found that an amountequivalent to 0.0334 gram of U0 had been deposited on the electrode in alayer less than 0.00005 inch in thickness. The electrode was then placedin a copper plating bath and 0.0665 gram of copper plated on to encasethe uranium oxide particle deposit and form a layer of uranium oxidedispersed in a copper matrix, in a layer substantially .00025 inch inthickness. This provided a uranium-to-Copper atom ratio of substantially1:8.5. The copper was deposited from a standard plating solution by aprocess Well known in the electroplating art, as described in MetalsHandbook, pages 716 et seq., 1948 Edition, published by the AmericanSociety for Metals, Cleveland, Ohio. The plating composition consistedof the following components per gallon of aqueous solution: coppercyanide 6 02.; total sodium cyanide 7.5 02.; free cyanide, NaCN, 0.7502.; Rochelle salt 6 02.; and sodium carbonate 4 oz. The plating Wascarried out at a tem perature of F., with a potential drop of 0.5 voltand a current of 0.05 amp for a period of about 2 /2 hours. After thecopper plating, the zirconium hydride electrode with the U0 and copperlayer thereon was removed from the solution, rinsed with distilledwater, dried with alcohol, and the alcohol permitted to evaporate.

EXAMPLE II The process of Example I was repeated with the modificationthat a layer of copper equivalent to 0.0071 gram was deposited on thezirconium hydride electrode prior to the uranium oxide deposition, andfurther that the U0 was electrophoretically deposited at 2500 volts. Anuclear reactor fuel element was obtained consisting of a zirconiumhydride moderator core with a layer of uranium dioxide fuel in theamount of 0.0334 gram uniformly dispersed in a copper matrix weighing0.0736 gram. The thickness of the U0 and copper was about 0.0002 inch ona zirconium hydride core having a diameter of 0.4001 inch. The atomratio of uranium-tocopper was substantially 129.4.

EXAMPLE III The procedure of Example I is repeated with the modificationthat the amount of copper deposited is 0.0079 grain and the amount of U0is 0.0334 gram, deposited in the form of particles of from about 0.3 toabout 100 microns in diameter, providing a uranium-to-copper atom ratioof substantially 1:1.

EXAMPLE IV The procedure of Example I is repeated with the modificationthat a layer of copper equivalent to 0.028 gram is deposited on thezirconium hydride core prior to the U0 deposition. This provides anover-all uranium-tocopper atom ratio of 1:12.

EXAMPLE V The procedure of Example 11 was repeated with the modificationthat the cycle of depositing first copper and then uranium dioxide wasrepeated four times with a final deposition of copper so as to providesuccessive deposits of copper and uranium dioxide in the followingamounts: 0.0071 gram copper, 0.0334 gram U0 deposited at a potentialdifference across the cell of 1200 volts; 0.0665 gram copper, 0.0242gram U0 deposited at 1200 volts; 0.1487 gram copper, 0.0335 gram U0deposited at a potenial of 1500 volts; 0.0583 gram copper, 0.3079 gramU0 deposited at 2000 volts; 0.2816 gram copper electrolyticallydeposited at a potential difference across the cell of 0.5 volt over aperiod of 2%. hours. The zirconium hydride electrode with its U0 andcopper deposit was placed in a nickel plating cell containing asolution, as described on page 718 of the Metals Handbook, supra. Acoating of nickel in the amount of 0.0172 gram was plated out on thesurface of the last copper deposition to provide a protective layer0.0001 inch thick. The amount of U0 on the zirconium hydride electrodewas 0.3990 gram dispersed in a copper matrix weighing 0.4957 gram. Thetotal thickness of the copper, U0 and nickel was 0.0017 inch. The atomratio of uranium-to-copper was substantially 1:53.

The procedure of Example V is repeated with the modification thatsuccessive layers of copper and U0 are de posited to provide a layer ofU0 in a copper matrix substantially 0.1 inch in thickness.

EXAMPLE VI The procedure of Example V is repeated with the modificationthat the protective layer is composed of 0.172 gram of nickel and issubstantially 0.001 inch thick. Stainless steel caps are welded on ateach end of the fuel element as shown in the drawing.

In like manner, the procedure of Example VI is repeated with themodification that a nickel protective layer substantially 0.1 inch thickis provided.

6 EXAMPLE VII A zirconium hydride core as described in Example I iscovered with a layer of copper equivalent to 0.007 gram. A layer ofuranium carbide and molybdenum of a particle size in the range of fromabout 0.3 to 50 microns is then electrophoretically deposited on thecopper-coated zirconium hydride core from an isopropyl alcohol solutionas described in Example I in which the ratio by weight of uraniumcarbide to molybdenum is 0.5. The layer of uranium carbide andmolybdenum is equivalent to 0.375 gram of uranium carbide and 0.75 gramof molybdenum. The atom ratio of uranium-to-molybdenum is substantially121.3. A layer of copper equivalent to 0.01 gram is deposited over theuranium carbide and molybdenum. The element with the zirconium hydridecore is then placed in a close-fitting copper tube substantially 0.01inch thick and the copper hydrostatically pressed onto the element Witha sodium-potassium medium at a temperature of substantially 1600 C.Steel caps are brazed onto the copper to form an element as shown in thedrawing.

In the same manner, nuclear reactor fuel elements are preparedconsisting of a moderator core of graphite, a fuel of plutonium oxidedispersed in a matrix of vanadium providing a plutonium-to-vanadium atomratio of substantially 1:3; a moderator core of yttrium hydride, a fuelof thorium oxide dispersed in a matrix of molybdenum providing athor-ium-to-molybdenum atom ratio of substantially 1:7; a moderator coreof titanium hydride, a fuel of plutonium carbide dispersed in a matrixof magnesium; a moderator core of a germanium and yttrium mixture, afuel of hydrided uranium, zirconium and yttrium alloy and uranium oxidedispersed in a matrix of aluminum; a moderator core of beryllium oxidein a matrix of zinc, a fuel of thorium carbide and uranium oxidedispersed in a matrix of zirconium; a moderator core of a bismuthleadalloy, a fuel of plutonium carbide and uranium oxide dispersed in amatrix of iron, covered with a corrosion resistant layer of tin.

EXAMPLE VIII Fuel elements are prepared according to the proceduredescribed in Example V in which the zirconium hydride cores are 2.3inches long and have a diameter of 0.4 inch. An amount of U6 equivalentto 0.2 gram and an amount of copper equivalent to 0.25 gram is depositedon each element. Stainless steel tubing, having a wall thickness of0.005 inch and a diameter 0.002 inch less than the diameter of the fuelelement, is heated to 1200 F. in order to cause it to expand. The fuelelement is then inserted in the tubing and the steel tubingshrink-fitted by cooling. One end of the tubing has a cap welded onto itas shown in the drawing prior to the insertion of the fuel element. Thecap at the other end of the element is welded on after the tube isfitted around the zirconium hydride corecontaining element. Theindividual cylindrical fuel elements are assembled into bundles betweenparallel plates 2.5 inches by 36 inches in dimension and Ma inch thick.The plates have holes drilled therein for receiving the end caps of thefuel elements. An assembly of the elements between parallel plates isshown in Organic Moderated Reactor Quarterly Progress Report bearingnumber NAASR-2057, pages 39 et seq., available from the Office ofTechnical Services, US. Department of Commerce, Washington 25, DC. Thecenter-to-center spacing of the individual fuel elements is 0.6 inch.The assembled fuel elements are placed in a fuel box 2.8 inch x 2.9 inchrectangular cross section, of the type described in the text entitledoolid Fuel Reactors by Diedrich and Zinn, pages 699 et seq, 1958edition, published by Addison- Wesley Publishing Company, Inc., Reading,Massachusetts. An organic moderated reactor as described on pages 696 etseq. of the Solid Fuel Reactors text, supra, is loaded with these fuelelements to provide enough ex- J cess reactivity to compensate for Xenonpoisoning, temperature, and fuel burnup. The reactor operatessatisfact-orily.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaim.

We claim:

A process for the manufacture of a nuclear reactor fuel elementcomprising providing a zirconium hydride core material, electrolyticallydepositing a layer of copper on said core, eiectrophoreticallydepositing uranium oxide on said copper, and electrolytically depositingcopper over said uranium oxide, in an amount such that the atom ratio ofsaid uranium to said copper is from about 1:1 to about 2,848,391 8/58Fahnoe et a1.

2,864,758 12/58 Shackelford 17683 2,872,388 2/59 Fahnoe et al 2041.52,894,885 7/59 Gray 2041.5 2,938,839 5/60 Fahnoe et al 2041.5 2,967,8111/61 Flint 17682 REUBEN EPSTEIN, Primary Examiner.

CARL D. QUARFORTH, Examiner.

